1. Related Field
The present invention relates to a unit for deaeration of drainage water from a paper machine, said unit comprising an inlet for receiving the drainage water in a first flow direction, an outlet for removing the drainage water to a subsequent deaeration channel, and a first chamber that is arranged downstream of the inlet, said first chamber comprising a first chamber part, and a second chamber part that is arranged downstream of the first chamber part.
The invention also relates to a method for deaerating drainage water, arriving from a forming roll in a wet end of a paper machine.
2. Description of Related Art
Normally, the drainage water flow is directed out of the wet end of the paper machine by first collecting the drainage water in troughs or channels. Thereafter, the drainage water is directed to a deaeration section. The deaeration section is constituted by one or several deaeration channels or conduits, so-called “flume”. These can be open or closed. In the following, such a deaeration section will be generally called a deaeration channel. In the deaeration channel, the drainage water moves at a flow velocity that allows a stable flow. In the deaeration channel, air bubbles in the drainage water are allowed to move up to the water surface and be released from the drainage water. The lower the flow velocity is, and the larger a free liquid surface the drainage water gets, the more efficient the deaeration will be.
At high machine speeds and high flows of drainage water, a very turbulent flow is produced, implying that the drainage water will have a high air content and thereby a large volume. The flow velocity of the drainage water in the wet end of the paper machine can be approx. 10-40 m/s. As an example, it can be mentioned that a paper machine with a width of approx. 3 m may produce a drainage water flow of approx. 40-70 m3/min, and a paper machine with a width of approx. 6 m may produce a flow of approx. 100-150 m3/min. When the drainage water is collected in troughs and channels in such operating situations, it may cause leakage, overflows and formation of waves to occur.
When rebuilding, or if it is desired to increase the capacity of an existing paper machine, problems may arise to make space for the required deaeration channel. Accordingly, it is desirable to produce an additional, efficient pre-deaeration, arranged in a position before the deaeration channels, which allows shorter deaeration channels to be used.
EP 1 424 437 A1 describes a known method of collecting drainage water from a forming roll in a twin-wire former of a paper machine. FIGS. 9 and 10 show a number of curved deflectors, which are positioned below the forming roll for directing the drainage water to a container, where the flow velocity of the drainage water is decelerated in connection with the twin-wire former.
U.S. Pat. No. 4,714,522 discloses an exemplary embodiment of how drainage water in a paper machine is decelerated. The drainage water is directed via a duct to a tower that is arranged adjacent to the paper machine. In the tower, the water is directed along the inside of curved wall portions which are arranged in the tower. The wall portions have openings through which the drainage water can pass. The water jets of drainage water passing through the openings hit the inner wall of the tower, and thereby the flow velocity of the drainage water is decreased.
U.S. Pat. No. 4,028,174 discloses a device for decelerating drainage water that is thrown from a rotating roll. A number of curved, perforated deflector plates are used for the deceleration of the water. The thrown water hits the concave surfaces of the deflector plates, and the water is forced out through the perforations by centrifugal force.
U.S. Pat. No. 6,096,120 discloses a deaeration chamber for removing air from a liquid (fibre suspension). The liquid is directed through an inlet pipe and then into straight, vertically rising passage pipes, said passage pipes opening into the chamber. After the liquid has left the passage pipes, the liquid hits the ceiling of the chamber and released air is evacuated from the chamber by a vacuum pump. The deaerated liquid is then directed to a vertical outlet, similar to a rain-water down pipe, for removing the deaerated liquid. A problem with this type of deaeration is that the liquid hits the deaerating surface (the ceiling of the chamber) with an impact and then falls down uncontrollably to the liquid level below. During this short time no efficient deaeration is achieved. Furthermore, the effective deaeration area that the liquid impinges on is very small, resulting in only a limited deaeration. This makes energy consuming vacuum pumps necessary to achieve a sufficient deaeration.